Field of the Invention
The present invention relates to an antenna circuit for a non-contact IC card and a method of manufacturing such an antenna circuit.
Description of the Related Art
Non-contact IC cards exchanging signals via electromagnetic waves or light have been used in recent years. FIG. 5 shows the configuration of such a non-contact IC card. A ROM 2 and a RAM 3 are connected through a bus 8 to a CPU 1 for controlling the operation of the IC card. An input/output control circuit 4 for controlling input of data from and output of data to an external device (not shown) is connected to the bus 8. An antenna circuit 6 is Connected to the input/output control circuit 4 through a modem circuit 5. A battery 7 is incorporated for supplying power to the individual electric circuits.
In such an IC card, a command signal received by the antenna circuit 6 from an external device, such as a terminal machine, in the form of an electromagnetic wave is demodulated by the modem circuit 5 and then input to the CPU 1 through the input/output control circuit 4. The CPU 1 decodes the command signal and creates a predetermined response signal. This response signal is input to the modem circuit 5 through the input/output control circuit 4 which modulates this signal. The modulated signal is transmitted to an external device from the antenna circuit 6.
In a practical IC card, as shown in FIG. 6, the CPU 1, the ROM 2, the RAM 3, the input/output control circuit 4, the modem circuit 5 and the bus 8 are fabricated in a single IC 9, and this IC 9 and the battery 7 are mounted on a card substrate 10. The antenna circuit 6 for exchanging signals with an external device has a coil 62 having an inductance L and consisting of a conductive pattern 61 formed in a spiral on the peripheral portion of the card substrate 10, and a capacitor 63 having a capacitance C and fabricated on the card substrate 10. A combination of the coil 62 and the capacitor 63 forms an LC parallel resonant circuit as shown in FIG. 7, which induces a voltage when it receives an electromagnetic wave at a frequency close to the resonant frequency of the resonant circuit. The signal is received, by detecting that induced voltage. In that case, the frequency of the electromagnetic waves that can be received by the antenna circuit 6 is determined by the resonant frequency fo=1/{2.pi.(L.multidot.C).sup.1/2 } of the LC parallel resonant circuit.
As stated above, the coil 62 of the antenna circuit 6 is formed on the peripheral portion of the card substrate 10 in the form of a conductive pattern. Therefore, the inductance L of the coil 62 is determined at the time of manufacture of the card substrate 10, and a change of the inductance L after the manufacture is impossible. Such an antenna circuit cannot be used when the frequency of a carrier used for exchanging signals is changed. Furthermore, matching of the resonant frequency cannot be conducted after the card substrate 10 is manufactured.